During processing of silver halide photographic materials, silver is often carried out of the photographic elements and into solutions, particularly the fix and bleach/fix solutions. As silver has more than a modest intrinsic value, many methods have been devised for desilvering these solutions to regain the metallic silver. Amongst the many available methods for reducing the silver in solution are electrolytic reduction, contact reduction with a metal, and chemical reduction. Each of these methods has its own advantages and disadvantages.
Electrolytic reduction produces high purity silver because it reduces ionic silver by merely supplying electrons to the silver ions in solution. It requires a large initial capital expenditure and therefore reduces the profitability of silver recovery. Electrolytic reduction is also ineffective at low concentrations of silver.
Chemical reduction and precipitation (as sulfide) also can produce high purity silver, but the cost of the most effective reducing agents can be significant. The particle size of the reduced silver or silver sulfide also can be quite small leading to difficulties in recovering the silver.
Metal contact reduction is an inexpensive means of reducing silver, but produces a low purity silver because of commingling with residual iron used in the oxidation/reduction process.
This metal displacement type of silver recovery process has traditionally been performed by passing silver rich solutions over iron, usually in the form of steel wool, wherein an electron exchange occurs between the active iron and the silver in solution. Iron is put into solution as ferrous ions and the silver is allowed to settle out. The iron metal Fe.degree. is oxidized mostly to ferrous ion and the silver ion Ag.sup.+ is reduced to silver metal. Because the iron or steel wool sits in an aqueous acidic solution during this process, it is subject to attack due to oxidation or hydrogen ion effects. Furthermore, the embrittlement of the steel wool in the acidic aqueous environment of spent fixer, such as fixer for X-ray, graphic arts, and other black and white fixers, tends to break down the filament structure and thus to destroy the electron pathway by which the silver ions in various parts of the solution are reduced by available iron wherever it may be found. This further decreases the percent utilization of all unoxidized iron.
When the stoichiometric balances between iron taken into solution and the amount of silver reduced were evaluated, it was found that in typical long term processing, whether continuous or intermittent in operation, much more iron was being brought into solution than would be expected from the metal displacement silver recovery reaction.